Many systems for controlling the steering of machines have included mechanical systems of which hydraulic pumps and various hydraulic valves and other components are used to control the steering of the vehicle. Likewise, electrically controlled four wheel steering systems have been used in which an electronic controller selects the steering mode and provides for independent control of the various steered wheels on the machine. In previous systems, spool type valves have been used but are subjected to various degrees of leakage between the spool and the bore in which the spool is slidably disposed. Likewise, in at least one instance, poppet valves have been used in an attempt to overcome the problems associated with spool type valves. However, in these systems, due to system operating parameters, it is difficult to keep the poppet valves positively seated during all operating conditions. If the poppet valves are not kept seated during operation, synchronization of the front and rear steered wheels is not maintained resulting in the steerable wheels drifting in directions other than that desired by the operator.
The present invention is directed to overcoming one or more of the problems as set forth above.